A conventionally-known Fresnel mirror includes a transparent planar plastic plate with a large number of annular inclined grooves concentrically formed in the back surface thereof, and a reflective film laid on the inclined grooves (see Patent Document 1, for example). The angle of inclination of the inclined grooves gradually increases as the distance of the inclined grooves from the concentric center in the radial direction increases. The Fresnel mirror can be made thin compared to conventional convex mirrors. Accordingly, even when a Fresnel mirror is placed on a wall surface of a passageway at an intersection such as a T-shaped, L-shaped, or cross intersection for confirmation of a blind spot near a corner of the passageway, the thin Fresnel mirror is not seen as an obstacle.
For example, in order to confirm the blind spot at a T-shaped intersection, a Fresnel mirror 3 is installed on a far end wall surface W1 at the end of a passageway P1 as illustrated in FIG. 11. When the Fresnel mirror 3 is installed in this manner, a person H1 who is moving straight along the passageway P1, which ends at the far end wall surface W1, looks at the Fresnel mirror 3 and can see the conditions of a blind spot S1 in the direction that the person H1 is going to turn at the corner. The person H1 can therefore avoid collision with a person H2, an object, or the like moving from the blind spot S1.
However, the Fresnel mirror 3 also reflects the conditions of a spot S2 in the direction that the person H1 is not going to turn. The person H1 who is going to turn left can visually know a person, an object, or the like moving from the spot S2 side without the Fresnel mirror 3 as long as the passageway P1 that the person H1 is moving along has a certain width. If the condition of the spot S2 in the direction that the person H1 is not going to turn is reflected on the Fresnel mirror 3, the person H1 may be sometimes confused by the reflected image. For example, in some cases, the person H1 does not know whether the image reflected on the Fresnel mirror 3 shows the conditions in the direction that the person H1 is going to turn or the conditions in the direction that the person H1 is not going to turn.
If such an unnecessary image is reflected on the Fresnel mirror 3, the person H1 looking at the Fresnel mirror 3 is confused and cannot clearly see the blind spot S1 in the necessary direction in some cases. The same problem occurs also in the case of the Fresnel mirror 3 is installed on a wall surface of a passageway at an L-shaped or cross intersection.
Moreover, in the case of using a single and entire Fresnel mirror 3, the Fresnel mirror 3 needs to be installed at a position of a widthwise center C2 of the passageway P1 ending at the far end wall surface W1 in order to reflect blind spots on both right and left sides in a passageway P2 extending to the right and left at the T-shaped junction. This makes smaller the blind spots on the both right and left of the passageway P2 which can be seen through the Fresnel mirror 3.